Solving a Coding Mystery

This was a surprising shock. This code seems so simple, but it does something very different than what I would expect.

var doc = new Dictionary<string,object>
{
    ["@metadata"] = new Dictionary<string, object>
    {
        ["@id"] = "users/1"
    }
    ["Name"] = "Oren"
};

Console.WriteLine(doc["Name"]);

The question is: Why?

As it turns out, we are missing one character here:

Notice the lack of the comma? Let's see how the compiler is treating this code by breaking it apart into steps, shall we?

First, let's break it into two statements:

var item = new Dictionary<string, object>
    {
        ["@id"] = "users/1"
    }
    ["Name"] = "Oren";
var doc = new Dictionary<string,object>
{
    ["@metadata"] = item
};

Note that we moved the name line into the first statement since there isn’t a command here. However, what is it actually doing? This looks very strange, but that is just because we have the dictionary initializer here. If we drop it, we get:

That makes a lot more sense. If we break it all down, we have:

var tmp1 = new Dictionary<string, object>();

tmp1["@id"] = "users/1";
tmp1["Name"] = "Oren";

var tmp2 = tmp1["Name"];

var tmp3 = new Dictionary<string, object>();

tmp3["@metadata"] = tmp2;

var doc = tmp3;

Console.WriteLine(doc["Name"]);

That explains it all. It makes perfect sense — and it's a very nasty trap. We ran into it accidentally in production code, and it was near impossible to figure out what was going on or why it happened.